Processes for preparing carbonyl compounds, e.g., aldehydes by hydroformylating an ethylenically-unsaturated precursor in the presence of a catalyst comprising rhodium hydridocarbonyl in complex combination with an organic ligand are well-known in the art and are now coming to be of increasing industrial importance. Typical of such processes is the hydroformylation of propylene to form butyraldehyde. These rhodium-based processes are now favored over the older technology wherein cobalt carbonyl is the major catalyst component for several reasons, including the fact that the rhodium systems can be used under relatively mild reaction conditions. Also, of very great importance, the rhodium-catalyzed systems can be controlled so as to yield a product in which the normal isomer of the aldehyde predominates over the branched-chain isomer to a greater extent than has normally been obtainable heretofore with the older reaction systems. It is to be understood that for most industrial purposes the normal aldehyde is strongly preferred over the branched-chain isomer as in, for example, those systems in which an aldehyde is initially formed, as by hydroformylation, and then oxidized to form the corresponding carboxylic acid which is then used as an intermediate in the production of synthetic lubricant base stocks. Considering heptaldehyde, for example, this compound is of very great importance as an intermediate in the production of heptanoic acid, certain esters of which are excellent base stocks for synthetic lubricant formulation, whereas the corresponding branched-chain acid is much less useful for this purpose.
More recently it has been discovered, as disclosed in Belgian Pat. No. 840906 (Oct. 20, 1976) and British Pat. No. 1,402,832, that bidentate ligands are particularly useful, with Belgian Pat. No. 840906 in particular disclosing that certain bidentate ligands which are derivatives of ferrocene are capable of yielding, under very moderate hydroformylation reaction conditions, an unusually high ratio of normal isomer to branched-chain isomer in the aldehyde product without the necessity of employing a high ratio of ligand to rhodium in the catalyst. More specifically, Belgian Pat. No. 840906 discloses that, with the ferrocene-based ligands, including specifically diphosphino-substituted ferrocenes, there is little need for maintaining in the reaction zone more than about 1.5 moles of the ferrocene derivative per atom of rhodium (equivalent to a phosphorus:rhodium mole ratio of 3.0:1). More recently yet it has been discovered, as set forth in U.S. Patent application Ser. No. 783,121 filed on Mar. 31, 1977 by J. D. Unruh and L. E. Wade, that there is another family of bidentate hydroformylation ligands which gives commercially attractive results similar to those of the ferrocene-based ligands, these newer ligands comprising cyclic compounds having in the ring two adjacent phosphinomethyl-substituted carbon atoms which are in trans relationship to one another and between which the dihedral angle of the trans positions is from about 90.degree. to about 180.degree..
In view of the foregoing it can be seen that bidentate ligands, and particularly diphosphino ligands, have come to be recognized as an advance over the relatively simple ligands, normally monodentate, which until recently have been considered typical and entirely satisfactory.
The industry continues, however, to seek further improvement in these rhodium-complex hydroformylation catalyst systems for several reasons which include (a) the recognition that any measures for reducing reaction pressure and temperature without suffering a loss in reaction conversion rate and normal:isoaldehyde ratio in the product will greatly reduce operating cost and (b) rhodium and the ligands both being costly, anything to improve catalyst efficacy and catalyst longevity will reduce both operating costs and investment cost. It is also to be kept in mind, of course, that the supply of rhodium available throughout the world is limited, so that obtaining maximum productivity per unit amount of rhodium-based catalyst is in itself a matter of unusual importance.
It is, accordingly, an object of the present invention to provide an improved hydroformylation process employing catalysts comprising rhodium hydridocarbonyl in complex combination with bidentate organic ligands, in particular diphosphino ligands. It is a further object to provide new ligands for use in such rhodium-catalyzed hydroformylation processes, the use of which facilitates operation at lower catalyst concentrations than are required with prior-art ligands. It is a further object to provide a method of general applicability for improving the efficacy of a given ligand by incorporating into its molecule certain substituent moieties which it has now been found have the effect of improving its efficacy in hydroformylation reaction systems.
Other objects will be apparent from the following detailed specification.